Add support for custom column names

Fixes issue #26.

README.md

@@ -974,6 +974,26 @@ type Animal struct {
 }
 ```
 
+If your column names differ from the struct fields, you can specify them like this:
+
+```go
+type Animal struct { // animals
+    AnimalId     int64     `gorm:"column:beast_id; primary_key:yes"`
+    Birthday     time.Time `gorm:"column:day_of_the_beast"`
+    Age          int64     `gorm:"column:age_of_the_beast"`
+}
+```
+
+Note that if your primary key has a custom column name, you will still have to
+specify `primary_key`, even if your struct field is named `Id`:
+
+```go
+type Foo struct {
+    Id int64 `gorm:"column:foo_id; primary_key:yes"`
+}
+```
+
+
 ## More examples with query chain
 
 ```go

callback_query.go

@@ -41,6 +41,19 @@ func Query(scope *Scope) {
 			return
 		}
 
+		colToFieldMap := make(map[string]string)
+		if destType != nil && destType.Kind() == reflect.Struct {
+			for i := 0; i < destType.NumField(); i++ {
+				fieldName := destType.Field(i).Name
+				dbColumnName := ToSnake(fieldName)
+				settings := parseTagSetting(destType.Field(i).Tag.Get("gorm"))
+				if colName, ok := settings["COLUMN"]; ok && colName != "" {
+					dbColumnName = colName
+				}
+				colToFieldMap[dbColumnName] = fieldName
+			}
+		}
+
 		defer rows.Close()
 		for rows.Next() {
 			anyRecordFound = true
@@ -52,7 +65,11 @@ func Query(scope *Scope) {
 			columns, _ := rows.Columns()
 			var values []interface{}
 			for _, value := range columns {
-				field := elem.FieldByName(SnakeToUpperCamel(strings.ToLower(value)))
+				fieldName, ok := colToFieldMap[value]
+				if !ok {
+					fieldName = SnakeToUpperCamel(strings.ToLower(value))
+				}
+				field := elem.FieldByName(fieldName)
 				if field.IsValid() {
 					values = append(values, field.Addr().Interface())
 				} else {

main_test.go

@@ -553,3 +553,40 @@ func BenchmarkRawSql(b *testing.B) {
 		db.Exec(delete_sql, id)
 	}
 }
+
+type MappedFields struct {
+	Id   int64     `gorm:"column:mapped_id; primary_key:yes"`
+	Name string    `gorm:"column:mapped_name"`
+	Date time.Time `gorm:"column:mapped_time"`
+}
+
+func TestMappedFields(t *testing.T) {
+	col := "mapped_name"
+	db := db.Model("")
+	scope := db.NewScope(&MappedFields{})
+	if !scope.Dialect().HasColumn(scope, "mapped_fields", col) {
+		t.Errorf("MappedFields should have column %s", col)
+	}
+	col = "mapped_id"
+	if scope.PrimaryKey() != col {
+		t.Errorf("MappedFields should have primary key %s, but got %q", col, scope.PrimaryKey())
+	}
+
+	expected := "foo"
+	mf := MappedFields{Id: 666, Name: expected, Date: time.Now()}
+	if !db.NewRecord(mf) || !db.NewRecord(&mf) {
+		t.Error("MappedFields should be new record before create")
+	}
+	if count := db.Save(&mf).RowsAffected; count != 1 {
+		t.Error("There should be one record be affected when create record")
+	}
+
+	var mfs []MappedFields
+	db.Table("mapped_fields").Find(&mfs)
+	if len(mfs) != 1 {
+		t.Errorf("Query from specified table")
+	}
+	if len(mfs) > 0 && mfs[0].Name != expected {
+		t.Errorf("Failed to query MappedFields, expected %q, got %q", expected, mfs[0].Name)
+	}
+}

migration_test.go

@@ -20,6 +20,7 @@ func runMigration() {
 	db.Exec("drop table animals")
 	db.Exec("drop table user_languages")
 	db.Exec("drop table languages")
+	db.Exec("drop table mapped_fields")
 
 	if err := db.CreateTable(&Animal{}).Error; err != nil {
 		panic(fmt.Sprintf("No error should happen when create table, but got %+v", err))
@@ -37,6 +38,10 @@ func runMigration() {
 		panic(fmt.Sprintf("No error should happen when create table, but got %+v", err))
 	}
 
+	if err := db.CreateTable(MappedFields{}).Error; err != nil {
+		panic(fmt.Sprintf("No error should happen when create table, but got %+v", err))
+	}
+
 	if err := db.AutoMigrate(Address{}).Error; err != nil {
 		panic(fmt.Sprintf("No error should happen when create table, but got %+v", err))
 	}

scope.go

@@ -258,6 +258,10 @@ func (scope *Scope) fieldFromStruct(fieldStruct reflect.StructField) *Field {
 
 	// Search for primary key tag identifier
 	settings := parseTagSetting(fieldStruct.Tag.Get("gorm"))
+	if colName, ok := settings["COLUMN"]; ok && colName != "" {
+		field.DBName = colName
+	}
+
 	if scope.PrimaryKey() == field.DBName {
 		field.IsPrimaryKey = true
 	}

utils.go

@@ -109,6 +109,9 @@ func GetPrimaryKey(value interface{}) string {
 
 			settings := parseTagSetting(fieldStruct.Tag.Get("gorm"))
 			if _, ok := settings["PRIMARY_KEY"]; ok {
+				if colName, ok2 := settings["COLUMN"]; ok2 {
+					return colName
+				}
 				return fieldStruct.Name
 			} else if fieldStruct.Name == "Id" {
 				hasId = true